REV. 0

ADP3170

–10–

Selecting a Standard Inductor

The companies listed in Table III can provide design consul-

tation and deliver power inductors optimized for high power

applications upon request.

Table III. Power Inductor Manufacturers

Coilcraft

(847) 639-6400

http://www.coilcraft.com

Coiltronics

(561) 752-5000

http://www.coiltronics.com

Sumida Electric Company

(408) 982-9660

http://www.sumida.com

current. The current comparator of the ADP3170 has a minimum

threshold of 69 mV. Note that this minimum value cannot be

used for the maximum specified nominal current, as headroom

is needed for ripple current and transients.

The current comparator threshold sets the peak of the inductor

equals the peak value less half of the peak-to-peak ripple current.

using the minimum current sense threshold of 69 mV yields:

R

V

I

I

mV

A

A

m

SENSE

CS TH

MIN

O MAX

RIPPLE

=

+

=

+

=

()(

)

()

.

.

2

69

23

59

2

266

Ω (5)

In this case, 2.5 m

Ω was chosen, assuming two 5 mΩ, 1 W

has been chosen, the output current at the point where current

current sense threshold of 87 mV:

I

V

R

I

I

mV

m

A

A

OUT CL

CS TH

MAX

SENSE

L RIPPLE

OUT CL

()

()(

)

(

)

()

–

.

–

.

.

=

==

2

87

25

59

2

31 6

Ω

(6)

At output voltages below 450 mV, the current sense thresh-

old is reduced to 54 mV, and the ripple current is negligible.

Therefore, the worst-case dead short output current is reduced to:

I

V

R

mV

m

A

OUT SC

CS SC

SENSE

()

()

.

.

==

=

54

25

21 6

Ω

(7)

To safely carry the current under maximum load conditions, the

sense resistor must have a power rating of at least:

PI

R

A

m

W

R

O

SENSE

SENSE

=×

=

×

=

2

2

23

25

133

..

Ω

(8)

Output Resistance

Intel’s VRM 8.5 specification requires that the regulator output

voltage measured at the CPU pins drops when the output cur-

rent increases. The specified voltage drop corresponds to a dc

output resistance of:

R

VV

I

VV

A

m

OUT

ONL

OFL

O

==

=

–.

– .

.

∆

Ω

1 845

1 771

23

32

(9)

The required dc output resistance can be achieved by terminating

tion resistance that will yield the correct dc output resistance is:

R

nR

gR

m

mmho

m

k

T

I

SENSE

m

OUT

=

×

×

=

×

×

=

25

2 5

22

32

888

.

..

.

Ω

Ω

Ω (10)

Output Offset

Intel’s VRM 8.5 specification requires that at no load the output

voltage of the regulator module be offset to a higher value than

the nominal voltage corresponding to the VID code. The offset

is introduced by realizing the total termination resistance of the

ground. The resistive divider introduces an offset to the output

allowed maximum at light load. Furthermore, the output of the

the current sense threshold is increased by the peak of the ripple

current in the inductor and reduced by the delay between sens-

ing when the current threshold has been reached and when the

high side MOSFET actually turns off. These two factors are

VV

IR

n

VV

L

tR

n

VV

Am

VV

H

ns

m

V

GNL

ONL

L RIPPLE

OUT

I

IN

OUT

D

SENSE

I

GNL

=+

××

××

×

=+

××

××

×

=

()

–

..

–

–.

..

2

1

59

32

25

2

51 8

1

60

2 5

25

1 224

Ω

Ω

µ

(11)

can now be calculated assuming that the internal resistance of

R

V

VV

R

gV

R

V

VV

k

mmho

mV

k

B

REF

REF

GNL

T

m

B

=

×+

=

×

=

–

–

–.

.

–.

.

3

3

1 224

888

22

45

29 7

Ω

Ω

(12)

Ω.